Paracrine Placental Growth Factor Signaling in Response to Ionizing Radiation Is p53-Dependent and Contributes to Radioresistance

The radiopharmaceutical radium-223 has immunomodulatory effects in patients and facilitates anti-programmed death receptor-1 therapy in murine models of bone metastatic prostate cancer

BACKGROUND & PURPOSE

Radium-223 (Ra223) improves survival in metastatic prostate cancer (mPC), but its impact on systemic immunity is unclear, and biomarkers of response are lacking. We examined markers of immunomodulatory activity during standard clinical Ra223 and studied the impact of Ra223 on response to immune checkpoint inhibition (ICI) in preclinical models.

MATERIALS & METHODS

We conducted a single-arm biomarker study of Ra223 in 22 bone mPC patients. We measured circulating immune cell subsets and a panel of cytokines before and during Ra223 therapy and correlated them with overall survival (OS). Using two murine mPC models-orthotopic PtenSmad4-null and TRAMP-C1 grafts in syngeneic immunocompetent mice-we tested the efficacy of combining Ra223 with ICI.

RESULTS

Above-median level of IL-6 at baseline was associated with a median OS of 358 versus 947 days for below levels; p = 0.044, from the log-rank test. Baseline PlGF and PSA inversely correlated with OS (p = 0.018 and p = 0.037, respectively, from the Cox model). Ra223 treatment was associated with a mild decrease in some peripheral immune cell populations and a shift in the proportion of MDSCs from granulocytic to myeloid. In mice, Ra223 increased the proliferation of CD8+ and CD4+ helper T cells without leading to CD8+ T cell exhaustion in the mPC lesions. In one of the models, combining Ra223 and anti-PD-1 antibody significantly prolonged survival, which correlated with increased CD8+ T cell infiltration in tumor tissue.

CONCLUSION

The inflammatory cytokine IL-6 and the angiogenic biomarker PlGF at baseline were promising outcome biomarkers after standard Ra223 treatment. In mouse models, Ra223 increased intratumoral CD8+ T cell infiltration and proliferation and could improve OS when combined with anti-PD-1 ICI.

Interfering with Tumor Hypoxia for Radiotherapy Optimization

Hypoxia in solid tumors is an important predictor of treatment resistance and poor clinical outcome. The significance of hypoxia in the development of resistance to radiotherapy has been recognized for decades and the search for hypoxia-targeting, radiosensitizing agents continues. This review summarizes the main hypoxia-related processes relevant for radiotherapy on the subcellular, cellular and tissue level and discusses the significance of hypoxia in radiation oncology, especially with regard to the current shift towards hypofractionated treatment regimens. Furthermore, we discuss the strategies to interfere with hypoxia for radiotherapy optimization, and we highlight novel insights into the molecular pathways involved in hypoxia that might be utilized to increase the efficacy of radiotherapy.